One type of conventional cutting element used in rotary drilling operations in earth formations comprises an abrasive composite mounted on a substrate. The composite typically comprises a body of sintered polycrystalline diamond material adhered to a cemented carbide substrate, such as cemented tungsten carbide, and containing a metal binder such as cobalt. Cutter inserts for machining and other tools also may comprise a body of polycrystalline diamond (PCD) bonded to a cemented carbide substrate. PCD is an example of a superhard material, also called superabrasive material, which has a hardness value substantially greater than that of cemented tungsten carbide.
Components comprising PCD material are used in a wide variety of tools for cutting, machining, drilling or degrading hard or abrasive materials such as rock, metal, ceramics, composites and wood-containing materials. PCD comprises a mass of substantially inter-grown diamond grains forming a skeletal mass, which defines interstices between the diamond grains. PCD material comprises at least about 80 volume % of diamond and may be made by subjecting an aggregated mass of diamond grains to an ultra-high pressure of greater than about 5 GPa, typically about 5.5 GPa or greater, and a temperature of at least about 1200° C., typically about 1440° C., in the presence of a sintering aid, also referred to as a catalyst material for diamond. A catalyst material for diamond is understood to be material that is capable of promoting direct inter-growth of diamond grains at a pressure and temperature condition at which diamond is thermodynamically more stable than graphite. Some catalyst materials for diamond may promote the conversion of diamond to graphite at ambient pressure, particularly at elevated temperatures. Examples of catalyst materials for diamond are cobalt, iron, nickel and certain alloys including any of these. PCD may be formed on a cobalt-cemented tungsten carbide substrate, which may provide a source of cobalt catalyst material for the PCD, the metal binder/catalyst from the cemented carbide substrate sweeping from the substrate through the diamond grains to promote sintering of the diamond grains. As a result, the diamond grains become bonded to each other to form a polycrystalline diamond layer and the diamond layer becomes bonded to the substrate. The interstices within PCD material may at least partly be filled with the catalyst material.
One of the factors limiting the success of the polycrystalline diamond (PCD) abrasive cutters is the generation of heat due to friction between the PCD and the work material. This heat causes thermal degradation of the diamond layer. Thermal degradation causes damage to the PCD through two mechanisms. Firstly, differential thermal expansion between the binder, which is also known as a solvent-catalyst material, and the bonded diamond crystals can cause the diamond-to-diamond bonding to rupture. Such differential thermal expansion is known to occur at temperatures of more than about 400° C. Secondly, the solvent metal catalyst can cause undesired catalysed phase transformation changing the diamond back to a graphitic or amorphous form limiting the practical use of the PCD material to a temperature of about 750° C.
One known technique to improve the thermal stability of the PCD material involves removing the sintered PCD layer from the substrate, subjecting the PCD layer to a suitable process for removing the solvent-catalyst material, such as acid-leaching, and subsequently re-attaching it to the substrate. The PCD with solvent-catalyst removed has good thermal stability and is commonly referred to as a thermally stable polycrystalline diamond (TSP). This process extends the useful cutting life of a cutting tool incorporating TSP as the cutting element. However, a problem known to exist with such TSP is that it is difficult to achieve a good re-attachment of the TSP to a substrate that can then be fabricated into a tool. The reason for this is that, typically, PCD compacts are brazed into a tool body for use, for example, in a drill bit for drilling in subterranean formations. If a conventional braze is used to attach the TSP to the substrate forming the abrasive compact cutting element, when the abrasive compact is brazed to the tool the heat from the brazing may soften the bond between the TSP and the substrate causing the TSP to become loose and move out of alignment. Furthermore, the process of attaching the TSP to a substrate is typically performed at high pressure and high temperature (HPHT) where diamond is thermodynamically stable and where the temperature is high enough to achieve bonding between the TSP and the substrate. This thereby renders the reattachment process an expensive process.
Other solutions that have been proposed to attach a cemented carbide substrate to a TSP body include to re-infiltrate the TSP body with the cobalt from the substrate or to place an additional layer of cobalt at the interface between the substrate and TSP body and subject it to a thermal cycle. The problem with these methods is that the cobalt requires a high temperature to melt and infiltrate the interstices between the diamond grains forming the TSP body. The temperature required to melt the cobalt is around 1300° C. and, at this temperature at ambient pressure, diamond is unstable in the presence of cobalt. The cobalt causes graphitisation of the TSP body and introduces thermal damage. However, if an HPHT cycle is used to maintain the TSP in the diamond stable region at the high temperatures when bonding the TSP to the substrate, high costs are incurred.
In addition, diamond is very difficult to wet, making the attachment of diamond to a variety of substrates difficult. As TSP compacts are essentially composed only of diamond, they are difficult to bond to cemented tungsten carbide supports, for example.
There is a need to overcome or substantially ameliorate the above-mentioned problems through a bonding technique for bonding a body of PCD material to a substrate.